1. Field of the Invention
This invention relates to a method of producing a biaxially oriented sheet or film and an apparatus therefor, more specifically to a method of producing a biaxially oriented sheet or film, wherein a thermoplastic resin having a high rigidity such as polystyrene is subjected to simultaneous biaxial orientation in a horizontal direction to be formed into a resin sheet or a resin film, and an apparatus therefor, and particularly to a method of producing a resin sheet preferably having a thickness of about 0.1 mm or more and an apparatus therefor.
2. Description of the Prior Art
Heretofore, there have been known a tenter method and a tubular method as the simultaneous biaxial orientation methods used for the thermoplastic resin. According to the tenter method, the resin is oriented by use of a tenter in a direction of transfer and in a widthwise direction of the resin. However, this tenter method presents such disadvantages as the equipment required becomes large sized in construction and the trimming loss caused during tentering is high, and hence, the tubular method is generally desirable.
As for this tubular method, in recent years, a two-stage compressed-air orientation method has been developed and used for polypropylene, polyethylene and the like. As shown in FIG. 1, according to this method, molten resin is extruded into a tubular form from a die 1 vertically downwardly (or upwardly). Compressed air is fed to the interior of the tubular resin through a first compressed air feed pipe 2. The tubular resin is cooled and regulated in shape or sized by a cooling vessel 3 provided on the outer periphery of the tubular resin. Thereafter the tubular resin is clamped by first nip rolls 4, and then compressed air having pressure required for orientation is fed through a second compressed air feed pipe 5 to orient the tubular resin. The tubular resin is clamped again by second nip rolls 6 and subjected to heat treatment by a heating air ring 7 to stabilize the orientation of the resin. The tubular resin is thereafter flattened by a bubble guide member 8, while being heated by a heater 9, so as to prevent wrinkles, crazings and the like from occurring. The resin is then folded by third nip rolls 10, and thereafter, pleats formed at opposite ends thereof are cut away, and the resin is wound into a takeup roll 11.
The method of the type described has presented such disadvantages that polystyrene resin having a very high rigidity, or some other thermoplastic resin, which is to be formed into a sheet having a large thickness, cannot be oriented because cracks take place in a tubular resin, and cannot be used commercially because of wrinkles, sags, distortions and the like caused during folding and flattening.
When polystyrene resin or one of the other thermoplastic resins is subjected to simultaneous biaxial orientation by a one-stage compressed-air method, molten resin, which has been extruded into a tubular form, is once cooled, and thereafter, reheated. However, polystyrene and the like are very low in thermal conductivity, and hence, a fairly large reheating region has been required to keep the inner side and the outer side of the tubular resin at a uniform temperature, thus presenting the disadvantages that the apparatus should necessarily be increased in its height, the installation is limited in position, operation of the apparatus becomes difficult and the safety of operators is not desirably ensured. Further, even if the conventional tenter method is used for polystyrene resin or the like, the trimming loss is high in the same manner as aforesaid and is not desirable from the viewpoint of the production efficiency.